It frequently occurs that a door mirror projecting from the side of a vehicle for monitoring the rear view makes an obstruction when it passes through a narrow street or is put into a garage. An electrically retractable door mirror has been brought into wide use recently, which has switch-selectable normal and stored positions. The stored position refers to a door mirror retracted position; on the other hand the normal position refers to a door mirror use position.
An electrically retractable door mirror, which is advantageous in terms of handling, allows a driver to retract the mirror by switching when it may possibly make an obstruction on such an occasion as garaging and return it to the normal position by another switching.
Japan Laid-Open Patent 08-207663 (hereinafter referred to as the prior art) discloses a conventional control unit for an electrically retractable door mirror. FIG. 5 is shows the circuit described in the prior art, which will be explained referring to the figure.
As shown in FIG. 5, the control circuit of the electrically retractable door mirror of the prior art includes, a DC (direct current) motor M for driving, a pair of power input terminals 1 and 2, a bipolar electrolytic capacitor C1, resistors R1-R6, motor drive FETs (field effect transistors) T1 and T2 and PNP (positive-negative-positive) transistors T3 and T4. The layout of the control circuit is symmetrical vertically relative to the bipolar electrolytic capacitor C1. Of the resistors R1-R6, R1 and R2 are the base resistors of the transistors T3 and T4, R3 and R4 are the excessive current detecting resistors which detect the excessive current and turn on the PNP transistors T3 and T4, and R5 and R6 are the gate resistors of the motor drive FETs T1 and T2, respectively.
When a driver selects “retract”, a positive voltage is imposed on the power input terminal 1 and a zero voltage on the power input terminal 2, respectively. A charging current thus flows into the bipolar electrolytic capacitor C1 through the gate resistors R5 and R6. The voltage of the gate G of the motor drive FET T1 is lower than that of the source S and thereby this motor drive FET T1 is turned on, since the current flows downward through the gate resistor R5 on FIG. 5. The current flows through the path—power input terminal 1→motor drive FET T1→excess current detecting resistor R3→DC motor M→excess current detecting resistor R4→motor drive FET T2→power input terminal 2 and thereby the DC motor M rotates in the positive direction to retract the mirror. When the voltage imposed on the power input terminals is reversed, the DC motor will rotate in the opposite direction to reset the mirror into the normal position.
An excess current called lock current flows into the DC motor M if the retraction movement reaches the stroke end or is blocked under some circumstances. A large voltage drop occurs at the excess current detecting resistor R3. If the voltage across the excess current detecting resistor R3 exceeds the ON-state voltage between the base and the emitter of the PNP transistor T3, it will be turned on. Subsequently, the gate resistor R5 is shorted and the motor drive FET T1 is turned off to shut off the current, and thereby the DC motor M comes to a stop. The possible burnout will thus be prevented since the current is shut off by the instantaneous detection of the excess current of the DC motor M.
However, the control circuit described above is likely to present difficulty in determining the resistance of the excess current detecting resistor, when the difference between the operating current and the lock current is not sufficiently large. There is the fear that the DC motor M comes to a stop in the middle of operation due to a false detection while it is operating normally or the lock current is not detected even though it actually flows into the DC motor M.
Since the semiconductors susceptible to the surge are used for the switches SWs shutting off the current flowing into the DC motor M, the alternative high voltage resistance elements or two pieces of the protection elements are required so that these semiconductors may be protected against the outside surge or the surge generated by the DC motor M. It will be followed by the cost increase in production.
The object of the present invention to assess the technical problems described above is to provide a control unit for an electrically retractable door mirror, which is capable of detecting the excess current securely and shutting off the current supplied to the motor when the excess current flows into the motor.